Optical disc drives have been widely used which irradiate a laser beam onto a disc-shaped information medium configured as an optical disc medium to record/reproduce information thereon. In the optical disc drive, a axial-tracking control that performs high-accuracy focus control on the optical disc medium while responding to the surface fluctuation of the optical disc medium is conducted, as well as a radial-tracking control that performs high-accuracy tracking control of a laser-focused-beam spot with respect to the information recording track on which information is recorded and reproduced. In recent years, a higher density recording of the information and a higher speed (twofold or higher-fold speed) recording/reproduction of the information have been developed, which fact requires a further higher accuracy in those positioning controls. This is because the higher density recording of the information is accompanied by a shorter wavelength of the laser light source and a narrower information recording track on which the information is recorded and reproduced, and achievement of stable recording/reproduction of the high-density information requires higher-accuracy tracking control of the laser-focused-beam spot. For example, the axial-tracking accuracy and radial-tracking accuracy required by the physical standard of a HD DVD (High-Density DVD) using a blue-violet laser as the laser light source are as low as 80 nm or less and 14 nm or less, respectively. Since the higher-fold speed drive in the optical disc drive is achieved by raising the rotational frequency of the optical disc medium, it is necessary to provide a positioning control unit having a higher tracking performance corresponding to the increase in the fluctuation of acceleration of the optical disc medium itself to be tracked for the positioning. The present inventor has conducted analysis of such a positioning control unit, as will be described hereinafter.
In general, it is possible to improve the above tracking accuracy and performance by raising the loop gain of the positioning control system and raising the response frequency band of the control loop. However, there is a limit on the characteristic of the drive machinery system that drives the moving member, and there is a problem in that a sufficient accuracy or performance cannot be secured in many cases. As the technique of solving such a problem, a technique may be considered that reduces an error by using the regularity of surface fluctuation of the disc-shaped information medium or positional fluctuation of the recording track. More specifically, since the up-and-down movement of the disc surface or positional fluctuation of the disc track of the optical disc medium is incurred by the rotation of the optical disc medium, it may be assumed that those components are mainly in synchrony with the rotation of the optical disc medium. Thus, taking advantage of the periodicity of position fluctuation of the information recording position (target position) of the optical disc medium (target member), the tracking performance of the moving member (laser-focused-beam spot) with respect to the target member can be improved by using the position error signal obtained prior to a single rotation or several rotations.
Conventional technologies that use the technique of reducing the error by taking advantage of the fluctuation regularity are described in JP-1985-57085B1 (Patent Publication-1) and W-2002-237154A1 (Patent Publication-2), for example. These conventional technologies use a signal delaying device that accumulates and stores the position error signal with respect to the position fluctuation having a constant period, input the instantaneous position error signal to the signal delaying device after adding the same to the output of the signal delaying device, and input the poison error signal to the drive unit of the moving member after adding the same to the output of the signal delaying device. In this way, these conventional technologies improve the tracking performance of the moving member with respect to the up-and-down movement of the disc surface or the position fluctuation of the disc track, which is in synchrony with the rotation of the disc-shaped information medium.
Here, the up-and-down movement of the disc surface and positional fluctuation of the disc track mainly include not only the components that are in synchrony with the disc rotation, but also components that are out of synchrony with the disc rotation, and these asynchronous fluctuations are also a factor of degrading the tracking accuracy and performance of the positioning control system. The up-and-down movement of the disc surface and positional fluctuation that are asynchronous with the disc rotation include a fluctuation that originates from the natural vibration of the target member that is generated due to excitation of the natural vibration mode by an external disturbance that acts particularly to the optical disc medium. Although the conventional techniques described in the above Patent Publication-1 and Patent Publication-2 are effective to the positional fluctuation that is synchronous with the rotation of the disc-shaped information medium, it is difficult to improve the tracking performance of the moving member with respect to the components of the positional fluctuation that is asynchronous with the rotation of the disc-shaped information medium by using the conventional techniques of Patent Publication-1 and Patent Publication-2, which rather degrade the tracking performance on the contrary and raise a problem.